Imagine someone who wanted their body dumped into an active volcano when they died, in order to really be one with Earth.  Even if this cost tens of thousands of dollars, few people would dump a significant other, or divorce a spouse, for this. Sure it is a bit weird, but hardly a deal-breaker.  Yet people do commonly divorce spouses for wanting their body dumped in liquid nitrogen at a similar expense, to live again.  (Bryan Caplan is aghast.)  What is the difference?  Two possibilities:

1. Even though skepticism about whether cryonics will work is one of the main arguments against it, in fact people think there's a substantial chance cryonics might actually work.  This triggers an abandonment reaction, like your buying a one-way-ticket to a distant land from which you could never return.  And it creates uncertainty about whether you are actually dead, making it harder for loved ones to have closure after a funeral.  This is the reason my wife gives for intending to prevent my being frozen. 

2. Saying you want to do something weird for value or symbolic belief reasons is far less threatening than saying you want to do something weird for instrumental reasons.  Common social norms encourage acceptance of weird values and symbolic beliefs, as long as those don't much effect ordinary behavior.  But by saying your weird act is a much better way to achieve important ordinary goals, you are saying the rest of us are making a big mistake.